As with many industries, the drive towards economy has also effected the beverage industry and has resulted in certain real changes in the way in which beverages are bottled, distributed and dispensed. In the last decade the beverage industry has seen a switch from glass to plastic containers. Plastic containers may be made from any number of materials depending on the application. One material is polyethylene terephthalate, "PET". Two types of PET bottles that are commonly used are single trip and multi-trip bottles. Single trip bottles are those which are filled, used, and then discarded. Multi-trip bottles are collected and reused and must be washed before refilling.
PET bottles offer several advantages over glass. Their light weight reduces freight costs. When dropped onto a hard surface they do not shatter like glass and generally do not break. Conveyor wear caused by the containers in the packaging plant is also reduced. The closure is also generally reusable after a bottle has been opened.
Disadvantages are that PET bottles are easily scratched, and susceptible to chemical attack when they are washed. PET containers also do not tolerate conditions above 60.degree. C. Exposing them to higher temperatures than 60.degree. C. causes deformation and/or shrinkage of the bottles.
Recycling of PET containers was recently approved by the FDA in the USA to permit turning used containers into new ones. Because new resin costs 50 to 75 cents per pound, recycling used bottles is economically attractive. In time, glass soft drink bottles are expected to disappear from the market. This heightens the relevance of PET container processing even further.
The cleaning of PET bottles takes place over a series of steps using caustic immersion tanks and spray wash stages in a bottlewasher. In the wash tanks, product residue, dirt, labels and labeling adhesive are removed. Because the surface of PET bottles is hydrophobic, cleaning them is more difficult than glass bottles. Also, the lower washing temperature decreases the chemical activity of the bottle washing solution.
Two principle problems in the cleaning of PET bottles are touched on by Laufenberg et al., "Cleaning, Disinfecting, and Transporting Pet Returnables", Brew Bev. Ind. Int. 1, 40-4 (0 ref.) January, 1992.
In this article, the author outlines the susceptibility of the PET bottles to corrosion or hazing. Hazing results from the chemical etching of the surface of the PET container by the caustic present in the wash bath.
An especially challenging problem which affects reusable PET bottles is the occurrence of mold in returned bottles. To simply discard all bottles from which mold can not be removed is prohibitively expensive. Reject rates of 40 to 50% have occurred at certain times of the year in countries located in tropical climates.
The necessary cleaning temperature for PET bottles is 60.degree. C. or less due to the glass transition temperature of PET. If exceeded, PET bottles deform and shrink. The cleaning power of a bottle washing solution at 60.degree. C. is only one quarter that at 80.degree. C. Bottles returned with product residue, i.e., those bottles that have not been rinsed, are almost always contaminated with microbiological forms of life. The bottle washing solution eliminates the presence of microbiological forms of life such as bacteria, spores, molds, and yeasts present in the bottle. However, at the reduced temperature of 60.degree. C., molds often present a persistent problem in the cleaning and reuse of PET bottles.
PET bottles simply cannot be washed like glass. Glass bottles are normally washed at 80.degree. C. Glass can also be washed with a relatively high concentration of caustic. While glass may be washed with up to 5.0% caustic, as little as 1.5% caustic can cause hazing in a PET container. With glass the washing temperature, the caustic concentration, and the washing time may be adjusted to allow for variability within the environment. In contrast, PET containers cannot withstand high levels of any of these variables.
While various alternatives have been proposed such as lowering the level of caustic, there remains a need in the industry for compositions and methods which allow for the efficient cleaning of PET containers and multiple reuse events.